The Insulin Resistance Syndrome.
The prevalence of metabolic disturbance, collectively known as metabolic syndrome or insulin resistance syndrome, has reached an epidemic proportion in industrialized countries. The insulin resistance syndrome refers to a constellation of findings, including glucose intolerance, obesity, an altered lipid profile (dyslipidemia) and hypertension, that promote the development of type 2 diabetes, cardiovascular disease, cancer, polycystic ovarian disease, and other disorders. In all of these disorders, a central component of the pathophysiology is insulin resistance. The underlying causes of this syndrome are overweight/obesity, physical inactivity and a series of currently not yet well-defined genetic polymorphisms (reviewed in 1+2). Lifestyle interventions and pharmacological treatment of the pathologies of the syndrome are only partially efficient and new therapeutic approaches are urgently needed.
Insulin and Insulin Resistance.
Insulin is a hormone produced by β-cells in the islets of Langerhans in the pancreas. Insulin release is stimulated as blood glucose levels rise and glucose is removed from the blood by insulin dependent stimulation of glucose transporters located in the cell membranes of target tissue, in particular in adipose tissue, skeletal muscle and liver. Insulin exerts its biological effects by binding to and activating the membrane-bound insulin receptor (IR), thereby initiating a cascade of intracellular signaling events, which regulates multiple biological processes such as glucose and lipid metabolism, gene expression, protein synthesis, and non-metabolic processes such as cell growth and differentiation. The diverse effects of IR activation are mediated through a multicomponent signaling complex that assembles upon binding of insulin. Thus, the intrinsic protein-tyrosine kinase activity of IR results in autophosphorylation of several tyrosine residues followed by recruitment and phosphorylation of several intracellular protein substrates including IR substrate (IRS) proteins and Scr-homolgy-2 containing (Shc) proteins. This initiates the activation of two main signalling pathways: the phosphatidylinositol 3-kinase (PI3K)-AKT/protein kinase B (PKB) pathway, which is responsible for most of the metabolic actions, including translocation of the glucose transporter GLUT4 to the cell membrane and stimulation of glycogen synthesis, and the Ras-mitogen-activated protein kinase (MAPK) pathway, which regulates expression of some genes and cooperates with the (PI3K)-AKT pathway to control cell growth and differentiation (reviewed in 3-8).
The ability of insulin to stimulate glucose disposal vary continuously throughout a population of apparently healthy persons, and a difference of ≥600% exists between the most insulin-sensitive and the most insulin resistance persons. However, the third of the population that is the most insulin resistant is at a much greater risk of developing several abnormalities and clinical syndromes, including type 2 diabetes, cardio vascular diseases, hypertension, stroke, non-alcoholic fatty liver, polycystic ovary disease, and certain forms of cancer (reviewed in 9) Individuals are said to be ‘insulin resistant’, because their tissues behave as if there was insufficient insulin in the bloodstream as reflected by decreased insulin response and glucose uptake in liver, adipose tissue, and skeletal muscle. The first response to insulin resistance is a compensatory production and secretion of insulin to compensate for the body's decreased sensitivity, leading to hyperinsulinaemia. Thus, high insulin levels and a decreased responsiveness of tissue to the clearance of glucose from the bloodstream characterize insulin resistance. Insulin resistance is the primary event leading to a series of metabolic changes including compensatory hyperinsulinemia, dyslipidemia, decompensation of pancreatic beta-cells, and hyperglycemia (reviewed in 6-8).
Type 2 Diabetes and Insulin Resistance.
Type-2 diabetes (non-insulin-dependent diabetes) is a complex and heterogeneous disorder associated with an increased risk for mortality as well as morbidity. The incidence is steadily increasing and the disease presently affects more than 150 million people worldwide making it a major public concern. The disorder is a prototypic complex polygenic disease with a strong heritable component but is also heavily influenced by environmental factors such as e.g. obesity. The pathogenesis of type 2 diabetes involves progressive development of insulin resistance in liver and peripheral tissue accompanied by defective insulin secretion from pancreatic beta cells leading to overt hyperglycaemia (an abnormally high amount of glucose levels in blood). The first response to insulin resistance is a compensatory production and secretion of insulin to compensate for the body's decreased sensitivity, leading to hyperinsulinaemia and rendering the individual prediabetic. However, when the pancreas of an insulin resistant individual is unable to produce sufficient hormone to compensate for the increased demand, the β-cell mass will ultimately be exhausted and degenerate leading to hyperglycemia and overt type-2 diabetes (reviewed in 4 and 5). Thus, type 2 diabetes only develops in subjects that are unable to sustain the β-cell compensatory insulin response. These subjects have “susceptible” as opposed to “robust” islets—a condition determined by genetic and/or acquired factors, ex obesity (FIG. 14).
Identification of a peptide/protein that could restore glucose metabolism and treat insulin resistance hold great promise as new therapeutic targets in the potentially combined treatment of type 2 diabetes, metabolic syndrome and other diseases characterised by insulin resistance.
The SorCS1 Receptor.
SorCS1 is one of five members of the mammalian Vps10p-domain (Vps10p-D) receptor family, which also comprises Sortilin, SorLA, SorCS2, and SorCS3. They are all type-1 transmembrane receptors sharing the characteristic structural feature of an N-terminal Vps10p-D with high homology to Vps10p, a sorting protein in yeast (10). At present the physiological function(s) of the receptor family is unclear, but recent findings indicate that both Sortilin and SorLA play a crucial role as regulators of neuronal survival and death (11,12). Interestingly, Sortilin has also been associated with insulin-regulated glucose up-take as it may facilitate translocation of the glucose transporter GLUT4 from an intracellular compartment to the plasma membrane (13,14).
SorCS1 is unique among the Vps10p-D receptors as it exists in several distinct splice variants, denoted SorCS1-a, b, c, c+, and d, that encode identical extracellular and transmembrane parts, and cytoplasmic domains that differ in length and sequence (10, 11). The present inventors, and others have found that SorCS1, in addition to in the nervous system, is expressed in adipose tissue, skeletal muscle and β-cells of the pancreas; all tissues involved in glucose metabolism. Moreover, each splice variant exhibit a distinct tissue distribution as well as subcellular expression pattern suggesting that the tail-variants might be implicated in different biological activities (15-17).